In a large bowl, combine flour and brown sugar. Cut in butter until crumbly. Stir in the oats. Press half into a greased 13-in. x 9-in. baking dish; top with rhubarb., In a small saucepan, combine sugar and cornstarch; stir in water until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat; stir in vanilla. Pour over rhubarb. Sprinkle with gelatin powder. Top with remaining crumb mixture., Bake at 350° for 40-45 minutes or until rhubarb is tender and topping is golden brown. Serve with ice cream if desired.